Multi-Tone Siren

These
multi-frequency signals are mixed and fed to the audio amplifier built
around IC LM386. The output of IC2 is fed to the speaker through
capacitor C9. If you want louder sound, use power amplifier TBA810 or
TDA1010. Only five outputs of IC1 are used here as the other five
outputs (pins 4 through 7 and 14) produce ultrasonic signals, which are
not audible. Assemble the circuit on a general-purpose PCB and enclose
in a suitable cabinet. Regulated 6V-12V (or a battery) can be used to
power the circuit.

Opamp VHF FM Transmitter

The circuit has a few striking characteristics.
Firstly, unlike normal oscillators that contain transistors this one
does not have any inductors. Secondly, there is no need for a varicap
diode to do the FM modulation. The opamp is configured as a Schmitt
trigger with only a small amount of hysteresis. The output is fed back
via an RC circuit. In this way, the trimmer capacitor is continually
being charged and discharged when the voltage reaches the hysteresis
threshold. The output continually toggles as a consequence.

This
results in a square wave output voltage. With a 10-pF trimmer capacitor
the frequency can be adjusted into the VHF FM broadcast band 88-108
MHz). The frequency of the oscillator is stable enough for this. The
output voltage is about 6 Vpp at a power supply voltage of 9 V. The
transmitter power amounts to about 50 mW at a load of 50R. This is about
20 times as much as the average oscillator with a transistor. With a
short antenna of about 10 cm, the range is more than sufficient to use
the circuit in the home as a test transmitter.

Because
the output signal is not free from harmonics the use of an outdoor
antenna is not recommended. This requires an additional filter/adapter
at the output (you could use a pi-filter for this). The FM modulation is
achieved by modulating the hysteresis, which influences the oscillator
frequency. An audio signal of about 20 mVpp is sufficient for a
reasonable output amplitude. The package for the opamp is an 8-pin SOIC
(provided you use the version with he RD8 suffix). The distance between
the pins on this package is 1/20 inch 1.27 mm).

This is still
quite easy to solder with descent tools. If SMD parts are used for the
other components as well then the circuit can be made very small. If
necessary, a single transistor can be added to the circuit to act as
microphone amplifier. The power supply voltage may not be higher than 12
V, because the IC cannot withstand that. The current consumption at 9 V
is only 15 mA. As with all free-running oscillator circuits, the
output frequency of this specimen is also sensitive to variations of
the power supply voltage.

For optimum stability, a power supply
voltage regulator is essential. As an additional design tip for this
circuit, we show an application as VCO for, for example, a PLL circuit.
When the trimmer capacitor is replaced with a varicap diode, the
frequency range can be greater than that of an LC oscillator. That’s
because with an LC-oscillator the range is proportional to the square
root of the capacitance ratio. With an RC oscillator the range is equal
to the entire capacitance ratio. For example: with a capacitance ratio
of 1:9, an LC oscillator can be tuned over a range of 1:3.

With
an RC oscillator this is 1:9. For the second tip, we note that the
circuit can provide sufficient power to drive a diode mixer (such as a
SBL-1) directly. This type of mixer requires a local oscillator signal
with a power from 5 to 10 mW and as already noted, this oscillator can
deliver 50 mW. A simple attenuator with a couple of resistors is
sufficient in this case to adapt the two to each other.

Power MOSFET Bridge Rectifier

The
principle is simple: whenever the instantaneous value of the input AC
voltage is greater than the rectified output voltage, a MOSFET is
switched on to allow current to flow from the input to the output. As we
want to have a full-wave rectifier, we need four FETs instead of four
diodes, just as in a bridge rectifier. R1–R4 form a voltage divider for
the rectified voltage, and R5–R8 do the same for the AC input voltage.
As soon as the input voltage is a bit higher than the rectified
voltage, IC1d switches on MOSFET T3.

Just as in a normal bridge
rectifier, the MOSFET diagonally opposite T3 must also be switched on
at the same time. That’s taken care of by IC1b. The polarity of the AC
voltage is reversed during the next half-wave, so IC1c and IC1a switch
on T4 and T1, respectively. As you can see, the voltage dividers are
not fully symmetrical. The input voltage is reduced slightly to cause a
slight delay in switching on the FETs. That is better than switching
them on too soon, which would increase the losses.

Be
sure to use 1% resistors for the dividers, or (if you can get them)
even 0.1% resistors. The control circuit around the TL084 is powered
from the rectified voltage, so an auxiliary supply is not necessary.
Naturally, that raises the question of how that can work. At the
beginning, there won’t be any voltage, so the rectifier won’t work and
there never will be any voltage... Fortunately, we have a bit of luck
here. Due to their internal structures, all FETs have internal diodes,
which are shown in dashed outline here for clarity.

They allow
the circuit to start up (with losses). There’s not much that has to be
said about the choice of FETs – it’s not critical. You can use whatever
you can put your hands on, but bear in mind that the loss depends on
the internal resistance. Nowadays, a value of 20 to 50 mW is quite
common. Such FETs can handle currents on the order of 50 A. That sounds
like a lot, but an average current of 5 A can easily result in peak
currents of 50 A in the FETs.

The IRFZ48N (55 V @ 64 A, 16 mW)
specified by the author is no longer made, but you might still be able
to buy it, or you can use a different type. For instance, the IRF4905
can handle 55 V @ 74 A and has an internal resistance of 20 mR. At
voltages above 6 V, it is recommended to increase the value of the
8.2-kR resistors, for example to 15 kR for 9V or 22 kR for 12 V.